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MG MGB Technical - Starting Problem

About three years ago, I replaced the worn out Lucas alternator on my 79 MGB with a Saturn 110 amp. alternator. Two months ago, I replaced the battery (old one was not taking a charge and it was under warranty). The car was starting easily until a few days ago. I thought that the head lamps might have been left on so I jump started the car and hooked up a charger for about one hour. The next few days the car stated right up. Once again, the car will not start without a jump or boast. Once the car starts, it runs for about ten minutes (ideling) and promptly stops. It will not restart without another jump.
I have checked the battery with a DC volt neter and it only reads about 10 volts after it has been running at idle (900 rpm). When I replace the battery, I fitted a new neg ground cable and cleaned the chassis ground.

The only thing that I think might be causing this problem is, the cover that fits over the front of the starter has come off and I have not refitted it to date. Any help/ suggestions would be greatly appreciated.

Cheers, Davy
Davy Crocker

Hi there,

Most likely the alternator is gone, It would be best
to remove it from the car and have it tested.
James

Agree, sounds like alternator. 10VDC probably isn't enough to keep the ignition and possibly the fuel system going. Even with a dead battery an alternator will raise system voltage to 13VDC or so at 900 rpm.

Your battery likely won't take too many more deep discharges and it too may not accept a charge.

Surprised your ALT warning lamp didn't glow.
Robert McCoy

You might try getting an aftermarket voltmeter to mount somewhere under the dash. There should be a white wire from the alternator that goes into the fuse block to power the fuel pump and ignition. It's switched. There might be an unused terminal on the fuse block that is part of that system and you can just use a spade terminal. If you tap into that wire, it will give you a readout on the voltage in the system as you drive. It will have a ticking in the needle that matches the fuel pump ticking. If there are any shorts in the system they'll show up. After the car has been setting, when you turn on the ignition switch, it will show the voltage of the battery and then after the engine starts it will show the output of the alternator. It will show problems that don't show up with the alt warning lamp.
Maybe that is helpful.
Ken
Kenneth Thompson

Is the alternator plugged in? As Robert says the warning light should glow if the alt is connected to the system but isn't charging (which at 10v it isn't). Whilst a single component or connection failure in the alternator could cause the warning light to stop working I don't think there are many situations where it prevents charging as well, or the warning light wouldn't be as much use as it is.
Paul Hunt 2

Is the Saturn alternator self exciting? If not, a burnt out alternator bulb in the dash can stop it from charging.
Test the output of the alternator. Make sure all the connections are clean and secure.
Kimberly

You can check the bulb by removing the plug from the alternator and grounding the brown/yellow (NOT the browns!) with the ignition on.

In practice if the warning light has failed used alternators are self-exciting, but the engine may have to be revved to about 3k to start charging. After that it should charge normally down to 600 rpm or so which is the point at which alternator cut out anyway. A brand-new alternator out of the box *may not* be self-exciting in this way.

Is *any* alternator self-exciting? And if so does that mean it doesn't have a warning light? If so that seems a retrograde step to me, as one would have to fit a voltmeter instead. I've seen a number of alternators described as 'single wire', but on investigation all it means is that *apart from* the warning light conenction, there is only one other connection to make i.e. to the main battery cable.
Paul Hunt 2

This thread was discussed between 11/09/2007 and 13/09/2007

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